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Stateside boffins say that, contrary to popular perception, it would often be better for the planet if people avoided using biodegradable products compliant with the recommended US government guidelines.

This is because biodegradable wastes – for instance cardboard cups, "eco friendly" disposable nappies, various kinds of shopping and rubbish bags etc – often wind up in landfill, where they will degrade and emit methane. Methane is, of course, a vastly more powerful greenhouse gas than CO2, so it is seen as important to prevent it getting into the atmosphere.

That's not always an issue, according to environmental engineering professor Dr Morton Barlaz. A lot of US landfills are already set up to capture their methane and either burn it off – so reducing the global-warming impact enormously – or pipe it out for subsequent use as fuel.

Unfortunately, however, landfills can't capture their gases until they are full up and capped off, and this takes time. And US Federal Trade Commission (FTC) guidelines call for products marked as "biodegradable" to decompose within "a reasonably short period of time" after disposal. Often a product marked "biodegradable" will decompose into methane in the depths of a landfill before the operator can get a cover over it.

"In other words," Barlaz says, "biodegradable products are not necessarily more environmentally friendly when disposed in landfills."

The answer, according to Barlaz, is to get away from the idea that rapid decomposition is always a good idea – especially on things which won't be recycled much but will probably wind up in landfill, for instance disposable nappies, fast-food packaging etc.

"If we want to maximize the environmental benefit of biodegradable products in landfills," Barlaz says, "we need to both expand methane collection at landfills and design these products to degrade more slowly – in contrast to FTC guidance."

The UK says it is meeting goals aimed at sending much less waste to landfill, but we still chuck nearly 15 megatonnes of rubbish into the ground each year. If we aren't sure that it is going to be recycled, we too should probably think twice about using biodegradable stuff.

Professor Barlaz' paper, Is Biodegradability a Desirable Attribute for Discarded Solid Waste? Perspectives from a National Landfill Greenhouse Gas Inventory Model is published by the journal Environmental Science & Technology. ®